Photochemical charge transfer reactions

Moorthy and Weiss (49) have also studied photochemical charge transfer reactions in frozen aqueous solutions. With I - and Fe +2 as the... 

Skvortzov VG, Myasnik MN, Sokolov VA, Morozov II (1981) Action spectrum for photoreactivation of Escherichia coli Bs.i after y-irradiation. Photochem Photobiol 33 187-190 Smith GJ (1979) The triplet-state charge-transfer reaction between p-nitroacetophenone and gu-anosine monophosphate. A possible mechanism for electron-affinic radiosensitization. Int J Radiat Biol 35 265-271... 

Importantly, all photoinduced processes share some common features. A photochemical reaction starts with the ground state structure, proceeds to an excited state structure and ends in the ground state structure. Thus, photochemical mechanisms are inherently multistep and involve intermediates between reactants and products. In the course of a photoinduced charge transfer reaction the molecule passes through several energy states with different activation barriers. This renders the electron transfer pathway quite complex. 

The participation of diradical species in charge transfer reactions has been demonstrated in the most widely recognized example of donor-acceptor interaction—i.e., the Diels-Alder reaction. The two-step nature of this reaction has recently been proposed in the isolation of both four-and six-membered ring products in the thermal and photochemical reaction of butadiene and a-acetoxyacrylonitrile (14, 56). 

Topics recently reviewed include photochemical electron-transfer reactions and exciplexes, the design of artificial photosynthetic systems, and photoredox reactions of excited states of phenanthroline complexes. The nonradiative relaxation of charge transfer (CT) excited states to their electronic ground states does not differ in any fundamental sense from other thermal electron-transfer processes. Such excited-state relaxation processes are conveniently viewed as a special class of intervalence transitions. Despite the general relevance of these excited state processes, only a few very pertinent reports are included in this survey. The coupling of CT excited-state relaxation times and CT emission maxima to solvent relaxation times has been noted. " ... 

Higher oligomers of the type M-Pt-M, where M is Fe, Ru, and Os have also been synthesized, and shown to undergo both thermal and photochemical charge transfer processes. " Electron transfer reactions in these complexes that involve more than one electron occur by a series of sequential one-electron transfer steps. Once the initial activation barrier for the first electron transfer has been overcome, the ensuing electron transfer steps generally occur in rapid succession. ... 

Irradiation of an ITIES by visible or UV light can give rise to a photocurrent, which is associated with the transfer of an ion or electron in its excited state. Alternatively, the photocurrent can be due to transfer of an ionic product of the photochemical reaction occurring in the solution bulk. Polarization measurements of the photoinduced charge transfer thus extend the range of experimental approaches to... 

Recently, photochemical and photoelectrochemical properties of fullerene (Cto) have been widely studied [60]. Photoinduced electron-transfer reactions of donor-Qo linked molecules have also been reported [61-63]. In a series of donor-Cfio linked systems, some of the compounds show novel properties, which accelerate photoinduced charge separation and decelerate charge recombination [61, 62]. These properties have been explained by the remarkably small reorganization energy in their electron-transfer reactions. The porphyrin-Qo linked compounds, where the porphyrin moieties act as both donors and sensitizers, have been extensively studied [61, 62]. 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

---